Recently, the performance and functionality of digital cameras and digital movie cameras that use some solid-state image sensor such as a CCD or a CMOS (which will be sometimes simply referred to herein as an “image sensor”) have been enhanced to an astonishing degree. In particular, the size of a pixel structure for use in an image sensor has been further reduced these days thanks to rapid development of semiconductor device processing technologies, thus getting an even greater number of pixels and drivers integrated together in an image sensor. As a result, the resolution of an image sensor has lately increased rapidly from around one million pixels to ten million or more pixels in a matter of few years. On top of that, the quality of an image captured has also been improved significantly as well. As for display devices, on the other hand, LCD and plasma displays with a reduced depth now provide high-resolution and high-contrast images, thus realizing high performance without taking up too much space. And such video quality improvement trends are now spreading from 2D images to 3D images. In fact, 3D display devices that achieve high image quality although they require the viewer to wear a pair of polarization glasses have been developed just recently.
As for the 3D image capturing technology, a typical 3D image capture device with a simple arrangement uses an image capturing system with two cameras to capture a right-eye image and a left-eye image. According to such a so-called “two-lens image capturing” technique, however, two cameras need to be used, thus increasing not only the overall size of the image capture device but also the manufacturing cost as well. To overcome such a problem, methods for capturing multiple images with parallax (which will be sometimes referred to herein as “multi-viewpoint images”) by using a single camera have been researched and developed. Such a method is called a “single-lens image capturing method”.
For example, Patent Documents Nos. 1 and 2 disclose a method for obtaining multi-viewpoint images by using two polarizers, of which the transmission axes cross each other at right angles, and a rotating polarization filter. Meanwhile, Patent Documents Nos. 3 to 5 disclose a method for obtaining multi-viewpoint images by using a diaphragm (light beam confining plate) with multiple color filters.
The methods disclosed in these Patent Documents Nos. 1 to 5 are used mostly to generate multi-viewpoint images using a single-lens camera. On the other hand, there is a technique for getting depth information using a single-lens camera with multiple micro lenses and for changing the focus position of the image captured freely based on that information. Such a technique is called “light field photography” and a single-lens camera that uses such a technique is called a “light field camera”. In a light field camera, a number of micro lenses are arranged on an image sensor. Each of those micro lenses is arranged so as to cover a plurality of pixels. By calculating information about the direction of incoming light based on the image information gotten through the image capturing session, the subject's depth can be estimated. Such a camera is disclosed in Non-Patent Document No. 1, for example.
The light field camera can calculate depth information. But its resolution is determined by the number of micro lenses and should be lower than the resolution determined by the number of pixels of the image sensor, which is a problem. Thus, to overcome such a problem, Patent Document No. 6 discloses a technique for increasing the resolution using two image capturing systems. According to such a technique, the incoming light is split into two divided incoming light beams, which are imaged by two image capturing systems, of which the groups of micro lenses are arranged so as to spatially shift from each other by a half pitch, and then the images captured in this manner are synthesized together, thereby increasing the resolution. However, this technique requires two image capturing systems, thus causing size and cost problems, too.
To overcome such a problem, Patent Document No. 7 discloses a technique for changing the modes of operation from a normal shooting mode into the light field photography mode, or vice versa, using a single image capturing system. According to this technique, a micro lens, of which the focal length varies according to the voltage applied, is used. Specifically, the focal length of the micro lens is set to be the infinity in the former mode and set to be a predetermined length in the latter mode. By adopting such a mechanism, an image with high resolution and depth information can be obtained. However, this technique requires a sophisticated control technique for controlling the focal length of the micro lens.
Meanwhile, Patent Documents Nos. 8 and 9 disclose techniques which were developed mainly for the purpose of getting depth information. According to these techniques, an image is captured through a diffraction grating which is arranged in front of a camera, and the distance from the subject to the diffraction grating is measured based on the magnitude of positional shift between an image produced by a zero-order diffracted light beam that has been transmitted through the diffraction grating (which will be referred to herein as a “zero-order light image”) and an image produced by a high-order diffracted light beam (which will be referred to herein as a “high-order light image”)